1. Field of the Invention
The present invention relates to an optical receiver used in optical communication, and more particularly, to an optical receiver capable of quickly establishing signal conduction in phase-modulated optical transmission.
2. Description of the Related Art
In recent years, in a wavelength multiplexing optical transmission system, in order to increase a transmission rate per channel while maintaining a wavelength interval between channels, it has been a challenge to increase a frequency utilization efficiency. Therefore, researches are being made to employ, instead of on-off keying (OOK) which have conventionally been used widely, a modulation format such as quadrature phase shift keying (QPSK) that provides narrow occupied bandwidth and high frequency utilization efficiency, for example.
Further, in the field of optical transmission, from the viewpoints of easy demodulation and optical fiber transmission characteristics, a return-to-zero differential quadrature phase shift keying (RZ-DQPSK) modulation format in which the QPSK modulation signal is differentially encoded and then RZ encoded is expected to be employed widely, for example.
As an example, in an optical receiver of the RZ-DQPSK modulation format, a delay interferometer (DLI) and a dual balanced receiver are used to add two quadrature phase components of received light waves with a delay of 1 time slot and then convert the added components to electric signals in two lanes that are proportional to amplitudes of the components for quadrature phase demodulation. Further, in order to regenerate one original data signal from the two electric signals, establishment of orthogonality of two lanes of the DLI and establishment of pattern matching (polarity inversion and lane swapping) of data signals associated with multilevel phase modulation of DQPSK are needed. Examples of the synchronization methods include methods disclosed in JP 2006-270909 A.